Re: design: function-exit probes

[Richard J Moore <richardj_moore at uk dot ibm dot com>]

Some points on reading your document:

1) why vote for having the return probe removal imply the probe removal.
Surely it should be the other way round.
2) is there a technical reason for enforcing removal of the trampoline
probe when the entry probe is removed? If not then I wouldn't impose this
restriction. Similarly vice versa if trampoline removal doesn't necessitate
probe removal.
3) the alternate implementation of placing a probe on the RA doesn't imply
the possibility of spurious hits as long at the probe is activated only
when the called function is entered.
4) I agree that some automatic actions will be needed. For example when a
called routine exits then the return probe will need to be removed
especially if it's implemented by altering the RA on the stack.
5) How are you going to handle multi-level returns? If you don't you'll
leak memory associated with the trampoline.
6) How will this work in a consistent way where the RA is stored in a
register e.g. zSeries and RISC architectures?
7) You don't get the coat-tailing problem with your approach but what about
dynamic return address generation? Not unreasonable where return and
decision logic (e.g based on success) is combined.

- -
Richard J Moore
IBM Advanced Linux Response Team - Linux Technology Centre
MOBEX: 264807; Mobile (+44) (0)7739-875237
Office: (+44) (0)1962-817072


                                                                           
             Jim Keniston                                                  
             <jkenisto@us.ib                                               
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             14/02/2005              design: function-exit probes          
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Here's my design for function-exit probes, inspired by a suggestion from
Will Cohen.  This doesn't reflect improvements or corrections that Hien
may be making as he implements it.

Sorry if this is a dup.  The first posting was rejected.

Jim Keniston




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from this note on 15 February 2005 by Richard J Moore